Thermally conductive flexible adhesive for aerospace applications

ABSTRACT

Provided are methods of forming thermally conductive flexible bonds for use in electronic boards of unmanned spacecraft and other types of aircraft. Also provided are methods of preparing adhesive materials to form these bonds including methods of preparing treated filler particles. In some aspects, an adhesive material includes filler particles having organofunctional groups, such as boron nitride particles treated in silane. These particles may be combined with a urethane modified epoxy to form the adhesive material. The weight ratio of the particles in the adhesive material may be about 40-60%. The adhesive material may be thermally cured using a temperature of less than 110° C. to prevent damage to bonded electronic components. The cured adhesive may have a thermal conductivity of at least about 2 W/m K measured in vacuum and may have a glass transition temperature if less than −40° C.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application claiming priority to U.S.application Ser. No. 15/258,073, entitled “THERMALLY CONDUCTIVE FLEXIBLEADHESIVE FOR AEROSPACE APPLICATIONS,” filed on Sep. 7, 2016, which is adivisional application claiming priority to U.S. application Ser. No.14/189,302, entitled “THERMALLY CONDUCTIVE FLEXIBLE ADHESIVE FORAEROSPACE APPLICATIONS,” filed on Feb. 25, 2014, issued as U.S. Pat. No.9,464,214 on Oct. 11, 2016, both of which are incorporated herein byreference in their entirety for all purposes.

BACKGROUND

Thermal management of electronic components and electronic boardsincluding these components is essential to the successful operation ofvarious aerospace vehicles, such as unmanned spacecraft. The continuedminiaturization of the electronic components and integration schemesresulted in a dramatic increase of heat generated per unit volume. Thisincreased heat generation not only limits the design of the circuit(e.g., the layout of electronic components on a board) and limits thedesign of individual electronic components but also jeopardizes thereliability of the overall circuit and individual components due tooverheating of the components, connections, conductive lines, and otherfeatures of the electronic boards.

The heat generated during operation of these electronic components needsto be transferred to other areas to ensure continuous operations of thecomponents. Because the boards often operate in vacuum environments,such as in unmanned spacecrafts, the heat may be primarily transferredthrough direct physical contact between various components, which may bereferred to as conductive heat transfer. Specifically, the heat istransferred from components to a board supporting these components, thenfrom the board to a chassis, and then from the chassis to a frame andother major components of the unmanned spacecraft.

Thermally conductive adhesives are often used to enhance the heattransfer between electronic components and boards supporting thesecomponents. A high thermal conductivity of these adhesives is essentialfor fast heat dissipation from the component and thermal management ofthe overall board. Currently available adhesives that meet specificrequirements for space applications have a thermal conductivity of onlyup to 0.6 W/mK. These adhesives use Al₂O₃ (alumina) as a conductivefiller suspended in an epoxy or polyurethane resin. Best thermallyconductive but electrically insulative adhesives, which are not spacecompatible, have a thermal conductivity of only about 1 W/mK. Theseadhesives employ a combination of alumina, boron nitride, and/oraluminum nitride as a conductive filler suspended in an epoxy,polyurethane or silicone resin.

It should be noted that these values of thermal conductivity are basedon measurements of a bonded joint configuration in a vacuum, which maybe viewed as a modified ASTM C 177 test method performed in vacuum.Specifically, an adhesive material is used to bond two aluminum plates,and the heat flux between these plates is measured to determine thethermal conductivity value of the adhesive material. This method isbelieved to be the most representative of heat transfer betweenelectronic components and boards supporting these components.

Other approaches used to measure a thermal conductivity include laserscanning of a free standing sample (e.g., a puck looking cylinder). Thevalues obtained from these other measurement techniques are oftendifferent or, more specifically, larger than values obtained using thebonded joint configuration in vacuum, often 2-50 times larger.Furthermore, these other measurement techniques are not performed invacuum resulting in artificially higher thermal conductivity valuescaused by additional heat losses through the air.

SUMMARY

Provided are methods of forming thermally conductive flexible bonds foruse in electronic boards of unmanned spacecraft and other types ofaircraft. Also provided are methods of preparing adhesive materials toform these bonds including methods of preparing treated fillerparticles. In some aspects, an adhesive material includes fillerparticles having organofunctional groups, such as boron nitrideparticles treated in silane. These particles may be combined with aurethane modified epoxy to form the adhesive material. The weight ratioof the particles in the adhesive material may be about 40-60%. Theadhesive material may be thermally cured using a temperature of lessthan 110° C. to prevent damage to bonded electronic components. Thecured adhesive may have a thermal conductivity of at least about 2 W/m Kmeasured in vacuum and may have a glass transition temperature of lessthan −40° C.

In some aspects, a method of forming a thermally conductive flexiblebond for use in an electronic board involves providing one or moreadhesive components. The one or more adhesive components may includeboron nitride particles having organofunctional groups attached to thesurface of these particles. Furthermore, the one or more adhesivecomponents may include a urethane modified epoxy. The urethane modifiedepoxy may be already premixed with the boron nitride particles and nofurther mixing may be needed. For example, the one or more adhesivecomponents may be provided as a frozen premix of the urethane modifiedepoxy and the boron nitride particles. Alternatively, the urethanemodified epoxy and the boron nitride particles may be provided asseparate components and mixed together during one of the lateroperations.

The method may proceed with forming an adhesive material from the one ormore adhesive components. This forming operation may involve warming upa frozen premix (a single frozen component) to an operating temperatureor mixing multiple components to form the adhesive material. Theadhesive material may be then applied onto a surface of the electronicboard by hand or by use of an automated dispensing system and a contactmay be formed between an electronic component and the adhesive materialapplied to the surface of the electric board. In some aspects, theadhesive material may be first applied to an electronic component andthen placed onto an electronic board.

The process may continue with curing the adhesive material, which formsa cured adhesive structure between the electric board and the electroniccomponent. The cured adhesive structure provides the thermallyconductive flexible bond between the electric board and the electroniccomponent. The flexibility, low glass transition temperature and lowcoefficient of thermal expansion of the cured adhesive structureprevents damage to the electric board and the electronic component whenthe assembly undergoes temperature changes (e.g., caused by theoperation of the electronic component) or when the electronic componentneeds to be replaced.

In some aspects, the concentration of the boron nitride particles in theadhesive material is between about 20% by weight and 70% by weight or,more specifically, between about 40% by weight and 60% by weight.Likewise, the concentration of the boron nitride particles in the curedadhesive structure between about 40% by weight and 60% by weight as nomaterials may be added or removed from the adhesive material when it iscured. The cured adhesive structure may have a thermal conductivity ofat least about 2 W/m-K or, more specifically, of at least about 3 W/m-K.Furthermore, the cured adhesive structure may have a glass transitiontemperature of less than about −40° C. or, more specifically, less thanabout −60° C. such as about −70° C. The cured adhesive material may havea shear strength of between about 100 psi and 500 psi. In someembodiments, the cured adhesive structure has a tensile modulus ofbetween about 10³ psi and 10⁵ psi. Furthermore, the cured adhesive has aweight loss of less than about 1% during outgassing.

The curing operation may be performed at a temperature of less than 110°C. such as about 100° C. At these low temperatures, the thermal damageto electronic components attached to the board is reduced. The durationof the curing operation may be between about 30 minutes and 120 minutes,such as about 60 minutes. The cured adhesive structure may have anaverage thickness of between about 0.001 inches and 0.010 inches betweenthe electric board and the electronic component. The cured adhesivestructure needs to be relatively thin to ensure low thermal resistance.Yet, the cured adhesive structure needs to physically contact both theelectronic component and the board and its thickness may be driven bythe spacing between electronic component and the board.

Also provided is a method of preparing a thermally conductive flexibleadhesive or, more specifically, a method of preparing silane treatedfiller particles. In some aspects, this method involves providing boronnitride particles. The average aspect ratio between any two dimensionsof the boron nitride particles may be less than 5. Such boron nitrideparticles may be defined as three dimensional particles with nodimensions being substantially larger (e.g., more than 5 times) than anyother dimensions. In some aspects, the average particle size of theboron nitride particles is between 10 micrometers and 200 micrometers.

The method may proceed with attaching hydroxyl groups to the surface ofthe boron nitride particles. The boron nitride particles having thehydroxyl groups on their surface are then exposed to a solution havingsilane. The silane may have organofunctional groups, such glycidylgroups or methoxy groups. The silane attaches to the surface of theboron nitride particles. The method may proceed with washing the boronnitride particles to remove residual silane not bound to theseparticles. Examples of suitable silanes include(3-glycidyloxypropyl)trimethoxysilane,[3-(2-Aminoethylamino)propyl]-trimethoxysilane, and(3-Trimethoxysilylproyl)-diethylenetriamine.

In some aspects, the solution containing the silane has a pH level ofbetween about 5 and 6. The weight ratio of the silane to boron nitrideparticles is between 2% by weight and 3% by weight. The concentration ofthe silane in the solution may be between about 0.08% by weight and0.35% by weight.

Also provide is a method of preparing a thermally conductive flexibleadhesive material. The method may involve providing a urethane modifiedepoxy and combining the urethane modified epoxy with boron nitrideparticles thereby forming a combined material. The concentration of theboron nitride particles in the combined material may be between about20% by weight and 70% by weight or, more specifically, between about 40%by weight and 60% by weight. The boron nitride particles may includeorganofunctional groups attached to a surface of the boron nitrideparticles. The method may proceed with mixing the combined material toform the thermally conductive flexible adhesive material. The mixing maybe performed using a dual asymmetric centrifugal mixer. The thermallyconductive flexible adhesive material may have a viscosity of at leastabout 100,000 cP after mixing. In some aspects, the method also involvesfreezing the thermally conductive flexible adhesive. Furthermore, priorto combining the urethane modified epoxy with the boron nitrideparticles, the method may involve mixing a base resin of the urethanemodified epoxy with a hardener of the urethane modified epoxy.

Provided also a thermally conductive flexible adhesive materialincluding a urethane modified epoxy and boron nitride particles or someother filler particles listed below. The concentration of the boronnitride particles or other filler particles may be between about 40% byweight and 60% by weight. In some embodiments, the boron nitrideparticles or other filler particles include organofunctional groupsattached to the surface of the particles. The thermally conductiveflexible adhesive material may be provided as a frozen premix or a kitof different components ready for mixing, e.g., using a dual asymmetriccentrifugal mixer. Prior to the application, the frozen premix may bebrought to the room temperature or the kit may be mixed. In someembodiments, the base resin and the hardener of the urethane modifiedepoxy provided in the kit are mixed together prior to introducing theboron nitride particles into the mixture. In the ready for applicationstate, the thermally conductive flexible adhesive material may have aviscosity of at least about 100,000 cP after mixing.

These and other aspects are described further below with reference tothe figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of an assembly including a thermallyconductive flexible adhesive bonding an electronic component to thesurface of an electronic board of an unmanned spacecraft, in accordancewith some aspects.

FIGS. 2A-2B illustrates a process flowchart corresponding to a method offorming a thermally conductive flexible bond for use in an electronicboard of an unmanned spacecraft diagram, which includes a method ofpreparing a thermally conductive flexible adhesive material andcomponents used in this adhesive material, in accordance with someaspects.

FIG. 3A-3C are schematic illustrations of filler particles at differentstages of the treatment process, in accordance with some aspects.

FIG. 3D is a schematic illustration of a thermally conductive flexibleadhesive material formed using the treated particles, in accordance withsome aspects.

FIG. 4A-4D are scanning electron microscope (SEM) photos of differentfiller particles.

FIG. 5A is a process flowchart reflecting key operations in the lifecycle of an aircraft from early stages of manufacturing and to enteringservice, in accordance with some aspects.

FIG. 5B is a block diagram illustrating various key components of anaircraft, in accordance with some aspects.

DETAILED DESCRIPTION

In the following description, numerous specific details are set forth inorder to provide a thorough understanding of the presented concepts. Thepresented concepts may be practiced without some or all of thesespecific details. In other instances, well known process operations havenot been described in detail so as to not unnecessarily obscure thedescribed concepts. While some concepts will be described in conjunctionwith the specific aspects, it will be understood that these aspects arenot intended to be limiting.

Introduction

As noted above, electronic components of aircraft and spacecraft oftenoperate in specific environments and/or operating conditions, whichcreate challenges in thermal management of these components. Forexample, unmanned spacecrafts operate predominantly in vacuumenvironments and have wide operating temperature ranges.

As such, electronic components of these unmanned spacecrafts heavilyrely on adhesives provided between these components and electronicboards supporting these components for heat dissipation. As such, a highthermal conductivity is essential for unmanned spacecraft applicationsand other aerospace applications. However, as also noted above, most ofspace-grade adhesives have a thermal conductivity of only up to 0.6W/mK, which significantly restricts design of electronic circuits. Inaddition to high thermal conductivities, space-grade and other types ofaerospace adhesives often need to be electrically insulating, have lowglass transition temperatures (e.g., lower than their operatingtemperature ranges), be sufficiently flexible to accommodate differentcoefficients of thermal expansion and temperature fluctuations, and havesubstantially no outgassing during their operation (e.g., in the vacuumenvironment and/or at large temperature variations). Each of theseproperties will now be described with reference to spacecraftapplications. However, one having ordinary skills in the art wouldunderstand that the described adhesive materials and methods ofpreparing these adhesives and using these adhesives to form thermallyconductive flexible bonds may be used for other aerospace applications.For example, some aircraft applications require adhesive materials withhigh thermal conductivities.

An adhesive material used to support an electronic component on a boardoften contacts different connecting terminals of this component as wellas connecting terminals of various other electronic components providedon the same board. For example, the adhesive material may be applied asa layer extending over the entire surface of the board and makingcontacts with all or most components on this board and, morespecifically, with their electrical connecting terminals. As such, theadhesive material uses for such applications needs to be electricallyinsulating. For purposes of this disclosure, an adhesive is deemed to beelectrically insulating when its resistivity is at least about 10¹⁰Ohm-cm. Yet, a combination of a high electrical resistivity and a highthermal conductivity is hard to achieve in the same material. Usually,materials with a high thermal conductivity are also good electricalconductors. For example, carbon-based materials are often used inthermally conductive adhesives, but these adhesives also tend to beelectrically conductive and may short the circuit.

Furthermore, for an adhesive material to be electrically insulating,each of its main components, such as an epoxy and filler particles,needs to be sufficiently insulating. Boron nitride, aluminum nitride,and other like materials are suitable filler particles as theirelectrical resistivity is typically greater than about 10¹⁴ Ohm-cm. Atthe same time, boron nitride and aluminum nitride have relatively highthermal conductivity as noted below. Finding a suitable electricallybinding agent is generally simpler.

Furthermore, space-grade adhesives generally need to have a very lowglass transition temperature, such as less than about −40° C. or evenless than about −50° C., such as about −70° C. Most commerciallyavailable thermally conductive adhesives have relatively high glasstransition temperatures, such as between +30° C. and +50° C. Such highglass transition temperatures are typically due to very high operatingtemperatures of these adhesives. However, unmanned spacecraft have muchwider operating range than many other applications. In some aspects, anunmanned spacecraft temperature range is between about −30° C. and +100°C. When a material goes through a glass transition, it may generatesignificant stress on bonded components causing various defects. Assuch, the glass transition temperature of an adhesive needs to beoutside of the operating range. More specifically, the glass transitiontemperature of an adhesive may need kept below this operating range. Assuch, most commercially available thermally conductive adhesives are notapplicable for unmanned spacecraft and other aerospace applications.

Another characteristic of space-grade adhesives is flexibility toprevent excessive stresses on electronic components and a board, inparticularly due to major temperature swings often associated withaerospace applications and different coefficients of thermal expansionof bonded materials. These stresses can damage the electroniccomponents, the board, and the connections causing cracking and otherinconsistencies. At the same time, flexibility needs to be limited toensure mechanical support to the bonded component. Flexibility may becharacterized by a tensile modulus and shear stress. For example, asuitable tensile modulus may be about 10³ psi and 10⁵ psi for unmannedaircraft and other aerospace applications. The shear strength of asuitable adhesive material may be between about 100 psi and 500 psi.These levels of the shear strength also allow removing and replacingcomponents (e.g., reworking an assembly after the adhesive material iscured) without damaging the electronic board and other components thatremain on the board.

Adhesive materials used for electronic boards of unmanned spacecraft andother aerospace applications often operate in environments that includeother critical components, such as other electronic components and/oroptical components. These other components may be highly sensitive tocontaminants and may become inoperable if outgassing products from theadhesive materials deposit on the surfaces of these other components.Furthermore, these operating environments are typically enclosed and/orhave low pressures (e.g., less than atmospheric pressure), which maycause more outgassing of the adhesive materials and more severeproblems. As such, cured adhesive materials for unmanned spacecraftapplications should produce substantially no outgassing. For purposes ofthis disclosure, a material produces substantially no outgassing whenits total mass loss is less than about 1% when tested in accordance withASTM E 595, entitled “Standard Test Method for Total Mass Loss andCollected Volatile Condensable Materials from Outgassing in a VacuumEnvironment.” The test is completed at 125° C. for 24 hours under a5×10⁻⁵ torr vacuum or better. Many adhesives that are not suitable foraerospace applications have high outgassing due to solvents, lowmolecular weight polymer, or excessive low molecular weight curingagents. The outgas data and other properties for various adhesives isprovided in Table 1 below:

TABLE 1 Storage Loss CTE, Shear Modulus, Modulus, ppm/C. strength,Outgass, % msi ksi Material Tg, C. <Tg >Tg psi TML CVCM VMR <Tg >Tg<Tg >Tg Aptek 95402 37.5 26 89 2413 0.761 0.047 0.076 0.77 0.03 31 1.03M TC2810 93 70 197 2699 0.873 0.016 0.293 0.33 0.009 11 0.09 Aptek 2311−77 35 133 434 0.662 0.019 0.299 0.69 0.02 27 1.5 Arctic Silver 52 39124 1857 4.75 0.004 0.110 0.51 0.0003 14 0.3

Provided are adhesive materials capable of forming thermally conductiveflexible bonds in electronic boards for use in unmanned spacecrafts andother aerospace applications. Also provided are methods of using theseadhesive materials to form such bonds and methods of preparing theseadhesive materials and various components thereof, such as treatedfiller particles. In some aspects, after curing, the adhesive materialshave a thermal conductivity of at least about 2 W/m K and even at leastabout 2.5 W/m K for bonded joint configurations tested in vacuum.Furthermore, these thermal conductivity values are even higher thangeneral thermally conductive adhesives that are not used for spaceapplications (and deficient in some of the above listed characteristics,such as outgassing, glass transition temperature, and othercharacteristics). It should be noted that these thermal conductivityvalues correspond to bonded joint configurations tested in vacuumreflective of unmanned spacecraft and other aerospace applications.

The described adhesive materials may have a glass transition temperatureof less than about −40° C. and, more specifically, less than about −60°C., such as about −70° C. Such a low glass transition temperatureensures that the operating temperature range does not overlap with theglass transition temperature thereby causing unexpected changes inmechanical properties during operation. Furthermore, these adhesivematerials are sufficiently flexible and produce substantially nooutgassing during their operation. In some aspects, the cured adhesivematerial has shear strength of between about 100 psi and 500 psiallowing the assemblies including these adhesives to be reworked withoutdamaging remaining components of the assemblies. Furthermore, thetensile modulus of the cured adhesive may be between about 10³ psi and10⁵ psi.

The described adhesive materials include filler particles and epoxiesmixed together. The filler particles may be formed from boron nitride,aluminum nitride, and/or other suitable materials. The filler particlesmay have organofunctional groups on their surface to enhance bondingwith the epoxies. For example, boron nitride particles may be treatedwith silane having organofunctional groups prior to combining theseparticles with a urethane modified epoxy. During curing the urethanemodified epoxy may form covalent bonds with the organofunctional groupson the surface of the filler particles. The weight ratio of theparticles in the adhesive material may be about 40-60% by weight. It hasbeen experimentally determined that this weight ratio provides themaximum thermal conductivity while maintaining mechanical and otherproperties for unmanned spacecraft and other aerospace applications.

The filler particles may be combined with the epoxy using dualasymmetric centrifugal mixing. This type of mixing utilizes simultaneousrotation and revolution movement of a mixing container and, in someaspects, may be performed in a reduced pressure environment to eliminateintroduction of air bubbles into the adhesive material. Furthermore, thedual asymmetric centrifugal mixing allows mixing of viscous materialswithout requiring dilution. In some aspects, the viscosity of the mixedadhesive material is at least about 100,000 cP or, more specifically, atleast about 500,000 cP. High-viscosity mixing capabilities allow largeweight ratios of filler particles and, in some aspects, provide highershear forces during mixing to ensure adequate dispersion of the fillerin the epoxy.

The described adhesive materials allow exploring new designs that havebeen previously limited by various power density constraints. Forexample, designs with tighter packaging of the components and/or designsusing more powerful electronic components may become available.Furthermore, additional functionality of the existing electroniccomponents may be explored. For example, some components may be operatedat higher frequencies and/or draw more power. Currently, capabilities ofsome components are purposely limited to prevent overheating of thesecomponents. In a particular example, the payload electronic currentlymay use 2 W rated components that are operated at 0.5 W due to thelimited heat dissipation capabilities with the currently used adhesives.

Examples of Electronic Assemblies Including Adhesive Materials

Prior to describing methods of forming thermally conductive flexiblebonds and methods of preparing adhesive materials to form these bonds, abrief description of electronic assemblies used for unmanned spacecraftsand other aerospace applications is presented to provide betterunderstanding of various features. Specifically, FIG. 1 is a schematicillustration of an assembly 100 including a cured thermally conductiveflexible adhesive 106 that bonds an electronic component 108 to surface104 of an electronic board 102. Electronic Board 102 may be installed onan unmanned spacecraft (e. g., Boeing 702HP, 702MP and 702SPspacecrafts). Examples of electronic components include resistors,transistors, diodes, capacitors, and other like devices. Electronicboard 102 may be made from polyimide and epoxy resins.

Cured adhesive 106 may form a layer on surface 104 of electronic board102. The layer may be a continuous layer or a patched layer.Specifically, the continuous layer may extend over surface 104 ofelectronic board 102 and extend in between adjacent electroniccomponents on this electronic board 102. On the other hand, the patchedlayer may only cover individual areas corresponding to footprints ofelectronic components. The patched layer may not extend in betweenadjacent electronic components. The average thickness of this layer maybe between about 0.001 inches and 0.010 inches or, more specifically,between about 0.004 inches and 0.008 inches, such as between about 0.005inches and 0.006 inches. This thickness covers various topographicalvariations on surface 104 of electronic board 102 and allows someseparation between the electronic component and the electronic board 102(e.g., to prevent electrical shorts). At the same time, an excessivethickness increases the thermal resistance of this interface and adds tothe overall weight of assembly 100.

In the final assembly, cured adhesive 106 directly contacts bothelectronic board 102 and electronic component 108 thereby providing apath for a heat flux between electronic board 102 and electroniccomponent 108. In some aspects, cured adhesive 106 may partially orfully encapsulate some electronic components. For example, an electroniccomponent may at least partially protrude into the layer formed by curedadhesive 106. Alternatively, an electronic component may have only asurface contact with a layer of cured adhesive 106 without protrudinginto the layer.

The characteristics of cured adhesive 106, such as its low shearstrength (e. g., less than 500 psi), may allow removal of electroniccomponent 108 from assembly 100 and replacing this original componentwith another component. For example, an original component may failduring testing or operation of assembly. This component may beelectrically disconnected from electronic board 102 and thenmechanically separated from cured adhesive 106 without disturbing othercomponents of the assembly. An additional uncured adhesive material maybe then introduced into this location and, when cured, may form athermally conductive flexible bond with the remaining cured adhesive anda new electronic component.

Processing Examples

FIGS. 2A and 2B illustrate a process flowchart corresponding to a method200 of forming a thermally conductive flexible bond for use in anelectronic board of an unmanned spacecraft and other aerospaceapplications, in accordance with some aspects. The thermally conductiveflexible bond is formed using a thermally conductive flexible adhesivematerial, which is often referred herein to as an adhesive material. Insome aspects, a method of preparing a thermally conductive flexibleadhesive material and/or components thereof is also a part of method200. Alternatively, a previously prepared adhesive material may be usedin method 200 without any upstream operations. For example, a frozenpre-mix may be provided, and method 200 may commence with operation 220during which the frozen pre-mix is brought to an operating temperature.

Furthermore, a method of preparing a thermally conductive flexibleadhesive may be performed without a subsequent use of this adhesive bythe same entity. For example, treated filler particles may be provided,and method 200 may commence with operation 212 and/or operation 214during which the treated filler particles are combined with an epoxy toeventually form an adhesive material. This adhesive material may be usedright away in operations 222-226 to form conductive flexible bonds ormay be frozen during operation 218 and stored to be used by the same ordifferent entity.

Furthermore, one or more components of a thermally conductive flexibleadhesive material may be prepared using various operations describedherein. These components may be used in a subsequent method (e.g., byanother entity) to prepare the adhesive material. For example, fillerparticles may be treated during operations 202-210 and then stored forfuture use by the same or different entity.

Overall, method 200 described herein with reference to FIGS. 2A and 2Bmay include different groups of operations performed independently fromeach other by the same entity or by different entities. As such, not alloperations presented in FIGS. 2A and 2B need to be a part of the samemethod.

In some aspects, method 200 may commence with providing filler particlesduring operation 202. The filler particles may be formed from boronnitride, aluminum nitride, aluminum oxide, and combinations thereof. Ingeneral, materials having a high thermal conductivity and a lowelectrical conductivity may be used as fillers for thermally conductiveflexible adhesive materials described herein. However, as notes above,most materials with high thermal conductivities are also good electricalconductors and, therefore, are not suitable as fillers. In some aspects,the filler particles are formed from boron nitride. Boron nitride has athermal conductivity of about 300 W/(m-K) (in the a-b plane of thecrystal), while its electrical resistivity is greater than about 10¹⁴Ohm-cm. In some aspects, the filler particles are formed from aluminumnitride. Aluminum nitride has a thermal conductivity of up to 140-180W/(m-K), while its electrical resistivity is greater than about 10¹⁴Ohm-cm. Of course, these properties may depend on purity, morphology,and other characteristics of the material.

The filler particles may be in a form of agglomerates, spheres, andplatelets. In general, the shape of the filler particles may bedescribed as a three-dimensional shape, such that an average aspectratio between any two dimensions of the filler particles is less than 5.The three-dimensional shape should be distinguished from aone-dimensional shape, in which one dimension substantially exceeds (bymore than 5 times) the two other dimensions. Some examples of theone-dimensional shape include wires, rods, fibers, and the like. Thethree-dimensional shape should be distinguished from a two-dimensionalshapes, in which each of two dimensions substantially exceeds (by morethan 5 times) the remaining dimension. Some examples of thetwo-dimensional shape include flakes, platelets, and the like. Thisunderstanding has been experimentally verified. Unlike the twodimensional particles or the one-dimensional particles,three-dimensional particles have comparable thermal conductivities inall three dimensions.

Specific examples of filler particles include PT350 boron nitrideparticles and PTX60 boron nitride particles, both supplied by MomentivePerformance Materials in Columbus, Ohio. PT350 particles have a meanparticle size of 125-150 micrometers, a surface area of 3.3 m²/g, andthe tap density of 0.7 g/m³. PTX60 particles have a mean particle sizeof 55-65 micrometers, a surface area of 5.5 m²/g, and the tap density of0.4 g/m³. An SEM image of PT350 particles is presented in FIG. 4D, whilean SEM image of PTX60 particles is presented in FIG. 4C. PT350 particlesmaybe characterized as medium-density agglomerates, while PTX60particles maybe characterized as spherical agglomerates.

Other examples of filler particles include PCTP30 boron nitrideparticles and PCTH3MHF boron nitride particles, both supplied bySaint-Gobain Ceramic Materials in Amherst, N.Y. PCTP30 particles have amean particle size of 75 micrometers, a surface area of 2.5 m²/g, andthe tap density of 0.8 g/m³. PCTH3MHF particles have a mean particlesize of 30 micrometers, a surface area of 2.5 m²/g, and the tap densityof 0.6 g/m³. An SEM image of PCTP30 particles is presented in FIG. 4A,while an SEM image of PCTH3MHF particles is presented in FIG. 4B. PCTP30particles maybe characterized as high-density agglomerates, whilePCTH3MHF particles maybe characterized as platelets.

In some aspects, the average particle size of the filler is betweenabout 10 micrometers and 200 micrometers or, more specifically, betweenabout 20 micrometers and 120 micrometers, such as between 50 micrometersand 150 micrometers. Similar to the shape of the filler particles, thesedimensions provide a specific combination of thermal and mechanicalproperties as further described below. Specifically, these dimensionsallow achieving a desirable packing density to provide a good thermalconductivity while retaining sufficient bonds between materials tomaintain mechanical properties.

Method 200 may proceed with pretreating the filler particles to attachhydroxyl groups to a surface of these particles during operation 204.For example, the filler particles may be treated in a high humidityenvironment, such as at a temperature 85° C. and 85% relative humidityfor four hours. The pretreated filler particles have demonstrated a muchhigher adsorption of silane on their surface as shown in theexperimental results section below. FIG. 3A is a schematic illustrationof a filler particle 300 prior to operation 204, while FIG. 3B is aschematic illustration of a filler particle 310 after operation 204showing hydroxyl groups attached to the surface.

Method 200 may proceed with exposing the filler particles to a silanecontaining solution during operation 206. Hydrolyzable groups of silaneundergo hydrolysis and condensation reactions as shown by the followingtwo reactions:

After the condensation reaction, the hydroxyl groups of the modifiedsilane form hydrogen bonds with the hydroxyl groups on the surface ofthe filler particles and may eventually form covalent bonding followingthe release of water. A schematic representation of a resultingstructure is shown in FIG. 3C. Specifically, structure 320 includes afiller particle 300 and covalently bound functional groups (R). In someaspects, these functional groups are organofunctional groups, such asepoxide groups (e.g., C₂OH₃R) or, more specifically, glycidyl groups,which include both epoxide and alcohol functional groups (e.g., C₂OH₃R,where R is O(CH₂)_(n), n=1-5 or, more specifically, n=2-4, such as n=3).A silane-containing solution used in operation 206 may also include asolvent. It has been found that some solvents, such as water, are moreeffective in forming bonds between silane and filler particles thanothers, such as methanol and isopropyl alcohol. A series of tests wereconducted to determine effectiveness of different solvents. For example,it has been found that effectively no silane was retained on the surfaceof boron nitride particles when methanol or isopropyl alcohol was usedto dissolve (3-glycidyloxypropyl) trimethoxysilane as further describedbelow.

Furthermore, it has been found that acidity of the solution effectsability of silane to bind to the surface of the filler particles. Forexample, no silane was bound to the surface of boron nitride particleswhen these particles were treated in a water-based solution of(3-glycidyloxypropyl)trimethoxysilane having a pH level of 3. Also, muchless silane was retained on the surface when the pH level was increasedabove a pH level of 6. In some aspects, the solution has a pH level ofbetween about 5 and 6. The solution may be a water-based solution inthese aspects.

The binding of silane to the surface of the filler particles may be alsoimpacted by the concentration of silane in the solution used duringoperation 206. In some aspects, the concentration of silane relative tothe weight of the filler particles (in this solution) is between about1.5% by weight and 4% by weight or, more specifically, between about 2%by weight and 3% by weight. Experiments have shown that 2.4% weightratio of (3-glycidyloxypropyl) trimethoxysilane to boron nitrideresulted in more silane bound to boron nitride particles than a 1.2% ora 4.8%. Too much silane may cause excessive cross-linking(polymerization) on the surface of the particles resulting inagglomeration of the particles. At the same time insufficient amounts ofsilane do not provide enough binding between the particles and theepoxy. It should be noted that while other components may be present ina solution used for treating filler particles with silane, the weightratios of silane are relative to the weight of the filler particlesunless other references are specifically stated.

In some aspects, method 200 may involve washing the filler particlesduring operation 208. The washing operation removes residual silane thatis not bound to the filler particles. For example, the treated particlesmay be mixed one or more times with water or some other suitable liquid.Method 200 may also involve an optional drying operation 210 duringwhich residual solvent may be removed from the treated particles. Insome aspects, the particles may be introduced into subsequent operationswith some residual solvent remaining if, for example, the solvent iscompatible with an epoxy used to form an adhesive material.

As noted above, method 200 may stop before operations 212 and 214 (e.g.,after completing operation 208 or operation 210). The treated fillerparticles may be stored at this point. In some aspects, the treatedparticles are transferred to another entity for further processing.Alternatively, method 200 may proceed with operations 212 and 214.

During operation 214 an epoxy or some other type of a base adhesivematerial is combined with treated particles thereby forming a combinedmaterial. A specific example of a base adhesive material is a urethanemodified epoxy. Polyurethanes, epoxies, and silicones, and urethanemodified epoxy may include two components, such as a resin and ahardener. The urethane modified epoxy used may be a low-modulus,low-glass transition temperature epoxy. Without adding filler particles(e.g., as a cured base adhesive material), the urethane modified epoxymay have a low glass transition temperature (e.g., less than −60° C.,such as about −80° C.) and low shear strength (e.g., less than 500 psi).

When a multi-component base adhesive material is used during operation214, the treated particles may be combined with one or both of thesecomponents. For example, the resin and hardener of an epoxy may be firstmixed during optional operation 212 and then this mixture may becombined with the treated particles during operation 214. The resin andhardener may be mixed using a dual asymmetric centrifugal mixer oranother type of mixer. When the dual asymmetric centrifugal mixer isused the mixing duration may be between about 15 seconds and 60 seconds,such as about 30 seconds. The rotation speeds may be between about 1000RPM and 5000 RPM, such as about 3000 RPM. At 3000 RPM rotation about themain axis of the mixer, the container also rotates at 750 rpm around itsown center axis. Overall, operation of the dual asymmetric centrifugalmixer will be readily understood by one having ordinary skills in theart.

Mixing the resin and hardener (prior to introducing the treatedparticles) reduces the viscosity of the mixture and allows introducingmore treated particles than otherwise possible. In some aspects, theconcentration (weight loading level) of the treated particles in thecombined material formed during operation 214 is between about 20% byweight and 70% by weight or, more specifically, between about 40% byweight and 60% by weight relative to the total amount of the combinedmaterial. These loading levels depend on the type of types, sizes,morphology and other characteristics of the filler particles.

Furthermore, mixing the resin and hardener (prior to introducing thetreated particles) allows reducing the mixing time needed for a mixturecontaining the treated particles and, therefore, reducing the overallstress experienced by these particles. It has been found that excessivemixing of materials containing particles tend to change the shape andsize of these particles. As a result, the thermally conductivecharacteristics of resulting adhesive material are often negativelyimpacted. For example, smaller particles may have less contact betweenthe particles than larger particles for the same weight loading. In someaspects, the total mixing time, after the treated particles areintroduced into the mixture, is less than 5 minutes or, morespecifically, less than 2 minutes, or even less than 1 minute. In someaspects, mixing is performed in stages with one or more cooling breaksin between the mixing stages. Controlling the temperature helps toreduce curing of the mixture at that stage. Other cooling techniques(such as using a cooling jacket on a mixing container) can be used aswell for this temperature control. For example, the temperature of themixture may be kept below 60° C. or, more specifically, below 50° C.during operation 214.

In some aspects, the treated particles may be combined with either theresin or the hardener but not both during operation 214. In other words,operation 212 is not performed. In some aspects, a portion of thetreated particles may be combined with the resin and another portion ofthe treated particles may be combined with the hardener. However, thesetwo portions may not be combined with each other until operation 214.These approaches eliminate the contact between the resin and hardenerand not initiate the curing process until later processing stages.

Method 200 may proceed with mixing the combined material to form athermally conductive flexible adhesive material during operation 216.The mixed adhesive material has a uniform distribution of the resin, thehardener, and the treated particles within the material unlike, forexample, the combined material at the end of operation 214. Afteroperation 216, the adhesive material may be ready for use or may bestored, for example, by freezing the adhesive during optional operation218. If the adhesive material is frozen, it may be brought back to anoperating temperature (e.g., a room temperature) during operation 220.

Operation 216 may involve mixing the combined material using a dualasymmetric centrifugal mixer or another suitable mixer. In some aspects,the rotational speed of the dual asymmetric centrifugal mixer is betweenabout 1000 RPM and 2000 RPM or, more specifically between about 1400 RPMand 1600 RPM. The lower speeds for this operation (as opposed theoperation used to pre-mix the resin and the hardener) is used to avoidgrinding down the filler particles. The thermally conductive flexibleadhesive may have a viscosity of at least about 100,000 cP after mixingor even at least about at least about 500,000 cP.

Method 200 may proceed with applying the adhesive material to a surfaceof an electronic board during operation 222 and forming a contactbetween an electronic component and the adhesive materials duringoperation 224. The adhesive material is then cured during operation 226.The curing operation may involve thermal curing, for example, by heatingthe assembly including the adhesive to less than about 110° C., such asto about 100° C., for about 1 hour. Many conventional electronic-gradeadhesives need curing temperatures of at least 120° C., which may bedamaging for components of unmanned spacecrafts. It should be mentionedthat adhesives containing fillers and adhesive used for applicationsthat does not allow direct line of sight generally cannot be cured usingexposure methods, such as UV curing, IR curing, X-Ray curing, and otherlike curing.

Experimental Results

Various experiments were conducted to characterize performance ofdifferent filler particles. Specifically, PT350 particles, PTX60particles, PCTP30 particles, and PCTH3MHF particles were tested. Somedescription and characteristics of these particles are presented above.In one test, a maximum possible weight loading of these four types ofparticles was tested using two epoxies, e.g., Aptek 95318, which has aglass transition temperature of less than −60° C. and is a urethanemodified epoxy system, and Aptek 95143, which has a lower viscosity thanAptek 95318 but has a higher glass transition of about 50° C. and is anepoxy system. Maximum loading is determined when the paste is no longerspreadable on bare aluminum and/or based on a maximum extrusion rate.PT350 particles had a maximum weight loading of about 62% in bothepoxies, while PTX60 particles had a maximum weight loading of about500/% in both epoxies. The average maximum loading was about 67% forboth for PCTP30 particles and PCTH3MHF particles. In general, thehighest possible loading is needed. However, there is a tradeoff betweenthe maximum loading, paste usability, and strength.

Another test was conducted to determine thermal conductivities of curedadhesive materials prepared using 30% weight loading of the same fourtypes of filler particles. A bonded joint configuration in vacuum wasused. Various aspects of this test are described above. A 5 mil bondline was controlled by adding 0.005″ diameter glass beads. No bondlinehad no control resulting in filler particle size (minor factor) andadhesive viscosity (major factor) controlling the bondline. The testresults are presented in TABLE 2A below.

TABLE 2A Thermal Conductivity (W/m K) at 30% Weight Loading Particles 5mil Bond Line No Bond Line PCTP30 0.65 0.10 PCTH3MHF 0.45 0.38 PT3500.79 1.05 PTX60 0.64 0.98

The thermal conductivity test was repeated for the same four types offiller particles (e.g., the PT350 particles, PTX60 particles, PCTP30particles, and PCTH3MHF particles) at their maximum loadings. Again, abonded joint configuration in vacuum was used. The test results arepresented in TABLE 2B below.

TABLE 2B Thermal Conductivity (W/m K) at the Maximum Weight LoadingsParticles Maximum Loading 5 mil Bond Line No Bond Line PCTP30 52% wt1.25 0.35 PCTH3MHF 55% wt 1.15 1.55 PT350 41% wt 1.49 3.90 PTX60 52% wt2.40 2.35

Yet another thermal conductivity test was conducted for variouscombinations of the PCTP30 particles and PCTH3MHF particles and,separately, for various combinations of the PT350 particles and PTX60particles. The total loading for these combinations was set at 30% byweight. The test results are presented in TABLE 3 and TABLE 4 below.

TABLE 3 Thermal Conductivity (W/m K) of Combinations of Two Types ofParticles PCTP30 Ratio PCTH3MHF Ratio 5 mil Bond Line  0% wt 100% wt 0.48 25% wt 75% wt 0.50 50% wt 50% wt 0.42 75% wt 25% wt 0.47 100% wt  0% wt 0.66

TABLE 4 Thermal Conductivity (W/m K) of Combinations of Two Types ofParticles PT350 Ratio PTX60 Ratio 5 mil Bond Line  0% wt 100% wt  0.6525% wt 75% wt 0.66 50% wt 50% wt 0.81 75% wt 25% wt 0.72 100% wt   0% wt0.79

Based on the above experimental results, PTX60 particles demonstratedthe best performance followed by PT350 particles. The surface area isbelieved to be the key factor in this test.

Additional tests were performed to determine parameters for treating thesurface of filler particles. The goal of this treatment was to increasecompatibility and wettability of particles with test epoxies. Onecompatibility aspect was to reduce a viscosity of the mixed adhesivematerial. Another aspect was establishing bonds between filler particlesand epoxies. Specifically, a silane treatment using(3-glycidyloxypropyl) trimethoxysilane was used. The silane treatmentfollowed by thermal gravimetric analysis (TGA), which involves heatingthe treated filler particles to 600° C. to decompose and remove most ofthe silane on the surface of the treated particles and to measure theweight loss as a result of this silane decomposition and removal.

To ensure bonding of silane to the surface of filler particles, thesurface was pre-treated to introduce hydroxyl groups as described above.Four different samples were subjected to the thermal gravimetricanalysis. The first sample included particles that were not exposed tosilane at all (e.g., the first reference). This sample demonstrated only0.08% weight loss attributed to non-silane losses/noise. The secondsample included particles that were exposed to silane but were notpre-treated (e.g., the second reference). This sample demonstrated about0.2% weight loss. The third sample included particles that werepretreated using sodium hydroxide and then exposed to silane. Thissample demonstrated about 0.1% weight loss. This value was notsignificantly greater than the value for the first reference and in factwas worse than the second reference. Finally, the fourth sample includedparticles that were pretreated in a humid environment (e.g., environmentcontaining a high concentration of water vapor) and then exposed tosilane. This sample demonstrated about 0.26% weight loss.

In another test different solvents were tested for a silane-containingsolution. (3-glycidyloxypropyl) trimethoxysilane was combined withdifferent solvents followed by treatment of the particles. Methanol andisopropanol demonstrated roughly the same performance as a referencesample that was not treated, which was less than 0.075% weight loss.However, when water was used to dissolve silane, the weight loss of thetreated filler particles went up to 0.26%.

Another test was conducted to determine effects of acidity of the silanecontaining solution on binding of silane to the filler particles. The pHlevels from 3 to 8.5 were tested. The results of this test are presentedin TABLE 5 below.

TABLE 5 TGA Weight Loss as a Function of Solution Acidity pH of SolutionTGA Weight Loss Reference (No Treatment) 0.075%  3 0.09% 5.5 0.26% 70.22% 8.5 0.21%

Yet another test was conducted to determine effects of the silaneconcentration on retention of the silane on the filler particles.Solutions with silane concentrations from 0% to 4.8% by weight based onthe weight of filler particles treated in these solutions were prepared,and the boron nitride particles were independently treated in each ofthese solutions. The results of this test are presented in TABLE 6below.

TABLE 6 TGA Weight Loss as a Function of Silane Concentration SilaneConcentration, by weight TGA Weight Loss Reference (No Treatment) 0.0750% (Solvent Only) 0.075 1.2% 0.16 2.4% 0.26 4.8% 0.13Examples of Aircraft

FIG. 5A is a general flowchart of manufacturing and service method 500for various vehicles, aircraft, and spacecraft. While the descriptionbelow refers to unmanned spacecrafts, one having ordinary skills in theart would understand that similar operations and general components maybe used for other vehicles, such as aircraft. FIG. 5B is a general blockdiagram of an unmanned spacecraft and will now be described to betterillustrate various features of processes and systems presented herein.

During pre-production, unmanned spacecraft manufacturing and servicemethod 500 may include specification and design 502 of unmannedspacecraft 530 and material procurement 504. The production phaseinvolves component and subassembly manufacturing 505 and systemintegration 508 of unmanned spacecraft 530. Thereafter, unmannedspacecraft 530 may go through certification and delivery 510 in order tobe used in service 512 (e.g., for a specific space mission). While inservice, unmanned spacecraft 530 may be scheduled for routinemaintenance and service 514 (which may also include modification,reconfiguration, refurbishment, and so on).

Each of the processes of unmanned spacecraft manufacturing and servicemethod 500 may be performed or carried out by a system integrator, athird party, and/or an operator (e.g., a customer). For the purposes ofthis description, a system integrator may include, without limitation,any number of unmanned spacecraft manufacturers and major-systemsubcontractors. A third party may include, for example, withoutlimitation, any number of venders, subcontractors, and suppliers.

As shown in FIG. 5B, unmanned spacecraft 530 produced by aircraftmanufacturing and service method 500 may include frame 532, and multiplesystems 534. Some examples of systems 534 include one or more ofpropulsion system 538 and electrical system 540. Electrical system 540may be fabricated and/or services using one or more of thermallyconductive flexible adhesive materials and methods described above.

Apparatus and methods disclosed herein may be employed during any one ormore of the stages of manufacturing and service method 500. Also, one ormore apparatus aspects, method aspects, or a combination thereof may beutilized during component and subassembly manufacturing 505 and systemintegration 508, for example, without limitation, by substantiallyexpediting assembly of or allowing new features (e.g., electroniccircuit designs) of unmanned spacecraft 530. Similarly, one or more ofapparatus aspects, method aspects, or a combination thereof may beutilized while unmanned spacecraft 530 is in service, for example,without limitation, to maintenance and service 514 may be used duringsystem integration 508 and/or maintenance and service 514 to determinewhether parts may be connected and/or mated to each other.

CONCLUSION

Although the foregoing concepts have been described in some detail forpurposes of clarity of understanding, it will be apparent that certainchanges and modifications may be practiced within the scope of theappended claims. It should be noted that there are many alternative waysof implementing the processes, systems, and apparatuses. Accordingly,the present aspects are to be considered as illustrative and notrestrictive.

What is claimed is:
 1. A method of preparing a thermally conductiveflexible adhesive material, the method comprising: pretreating boronnitride particles in a water-vapor containing environment, therebyattaching hydroxyl groups to surfaces of the boron nitride particles;exposing the boron nitride particles comprising the hydroxyl groups tothe surfaces of the boron nitride particles to a silane-containingsolution, thereby converting the hydroxyl groups to silane-containingorganofunctional groups, covalently bound to the surfaces of the boronnitride particles; combining a urethane modified epoxy with the boronnitride particles, comprising the silane-containing organofunctionalgroups, covalently bound to the surfaces of the boron nitride particles,thereby forming a combined material, mixing the combined material toform the thermally conductive flexible adhesive material.
 2. The methodof claim 1, wherein mixing the combined material is performed using adual asymmetric centrifugal mixer.
 3. The method of claim 1, furthercomprising freezing the thermally conductive flexible adhesive material.4. The method of claim 1, wherein the thermally conductive flexibleadhesive material has a viscosity of at least 100,000 cP after mixing.5. The method of claim 1, wherein the thermally conductive flexibleadhesive material has a viscosity of at least 500,000 cP after mixing.6. The method of claim 1, further comprising, prior to combining theurethane modified epoxy with the boron nitride particles, mixing a baseresin of the urethane modified epoxy with a hardener of the urethanemodified epoxy.
 7. The method of claim 1, wherein the concentration ofthe boron nitride particles in the combined material is between 40% byweight and 60% by weight.
 8. The method of claim 1, wherein mixing thecombined material comprises controlling temperature of the combinedmaterial.
 9. The method of claim 8, wherein the temperature of thecombined material is kept below 60° C. while mixing the combinedmaterial.
 10. The method of claim 1, wherein mixing the combinedmaterial is performed in stages, with a cooling break between twoadjacent stages.
 11. The method of claim 1, wherein an average particlesize of the boron nitride particles is between 50 micrometers and 150micrometers.
 12. The method of claim 1, wherein the organofunctionalgroups are selected from the group consisting of glycidyl groups andalcohol functional groups.
 13. The method of claim 1, wherein theorganofunctional groups and represented by a formula C₂OH₃R, where R isO(CH₂)_(n) and where n is between 1 and
 5. 14. The method of claim 13,wherein n in O(CH₂)_(n) is between 2 and
 4. 15. The method of claim 14,wherein n in O(CH₂)_(n) is
 3. 16. The method of claim 1, wherein thesilane, comprising the organofunctional groups, is (3-glycidyloxypropyl)trimethoxysilane.
 17. The method of claim 1, wherein theorganofunctional groups are covalently bound to the surface of the boronnitride particles.
 18. The method of claim 1, further comprising:applying the thermally conductive flexible adhesive material onto asurface of an electronic board; forming a contact between an electroniccomponent and the thermally conductive flexible adhesive materialapplied to the surface of the electric board; and curing the thermallyconductive flexible adhesive material thereby forming a cured adhesivestructure between the electric board and the electronic component. 19.The method of claim 18, wherein the cured adhesive structure provides athermally conductive flexible bond between the electric board and theelectronic component.
 20. The method of claim 18, wherein the curedadhesive structure at least partially encapsulates the electroniccomponent.